Abase station for a mobile communication system and microwave radio links used for data transport typically comprise one or more transceiver units connected to an antenna for transmitting and receiving microwave signals. These transceivers in turn comprise a diplexer/duplexer consisting of at least two band-pass filters. The filters of the diplexer may have different passbands so as to, e.g., prevent intermodulation between a transmission signal and a received signal. Herein, when referring to a passband of a filter, it is appreciated that a passband is defined by a center frequency and a bandwidth, the bandwidth being measured, e.g., when the return loss is lower than a certain level, such as −20 dB.
Microwave filters can be of the transmission line type, such as a microstrip arranged on a dielectric carrier. However, hollow metal waveguides are more often used as filters due to lower losses and a higher power capability compared to microstrip filters, even though a hollow waveguide filter will have a larger size than a microstrip filter.
The dimensions of a hollow waveguide filter are dependent on the frequency of the signal to be filtered, the selected filtering properties such as a certain passband, and on the type of filter used. Since the size of the waveguide must be on the same order as the wavelength of the frequency of the signal that is to be filtered, hollow waveguides are typically used for frequencies in the GHz range which have wavelengths in the mm range.
In some applications, such as in outdoor microwave radio or radio base station units, there are strict size limitations which must be adhered to. Thereby, the available space also dictates which type of filter can be used. It is therefore often desirable to reduce the size of a filter without degradation of the frequency properties of the filter. As an example, waveguide H-plane type filters are known to have advantageous frequency properties and they also can be made smaller than other comparable types of filters such as E-plane filters. However, H-plane filters require a large number of tuning positions making it costly and complicated to tune the filters.
A known alternative to H-plane filters are waveguide E-plane filters which do not need to be tuned. In an E-plane filter, a conductive foil or insert is arranged in the waveguide filter at or close to the location where the strength of the E-field (V/m) is the highest. The foil or insert comprises openings which act as resonators, thereby determining the poles of the filter, and consequently also contribute to determining the passband of the filter. However, an E-plane filter can not be made as small as an H-plane filter with the same filtering properties.
Accordingly, it is desirable to provide an improved waveguide filter which is both comparatively small in order to be used within a restricted space and also uncomplicated to manufacture, without degradation of the filtering properties.